Image sensors can be used in a variety of applications, such as digital still cameras, PC cameras, digital camcorders and Personal Communication Systems (PCS), as well as analog and digital TV and video systems, video game machines, security cameras and micro cameras for medical treatment. With the development of the telecommunication and computer system, the demand for image sensors will be much more increased.
An image sensor cell typically has a photodiode element, which is capable of converting light (e.g., visible light, infrared light and ultraviolet light) into electric signals. When photons are absorbed, electron-hole pairs are created through photoelectric conversion. A depletion region is formed in a photodiode when the photodiode is reverse-biased. The electric field in the depletion region separates the electron-hole pairs generated from photoelectric conversion.
The electric current generated from the photoelectric conversion can be directly measured to determine the intensity of the light. However, the signal generated from the direct measurement of the current from photoelectric conversion typically has a poor signal to noise (S/N) ratio. Thus, a typical image sensor accumulates the charges generated from photoelectric conversion for a predetermined period; and, the amount of accumulated charges is measured to determine the intensity of the light.
To measure the accumulated photoelectric charges, a CMOS (Complementary Metal-Oxide Semiconductor) Active Pixel Sensor (APS) contains active circuit elements (e.g., transistors) for measuring the signal associated with the accumulated photoelectric charges. Alternatively, the accumulated charges can be moved out of image sensor cell for measurement (e.g., in a CMOS Passive Pixel Sensor (PPS) or in a Charge Coupled Device (CCD) image sensor). In order to prevent noise, a CCD image sensor uses a complicated process to transfer the accumulated charges from the sensor cell to an amplifier for measurement. A CCD device uses complicated driving signals of large voltage swings, and thus, consumes a lot of power. While a CMOS PPS can be fabricated using a standard CMOS process, a typical CMOS PPS has a poor Signal to Noise (S/N) ratio. A typical CCD fabrication process is optimized for charge transfer; and it is not compatible with a standard CMOS process. Thus, a CCD image sensor is difficult to be integrated with signal processing circuitry, which is typically implemented by Complementary Metal-Oxide Semiconductor (CMOS) circuitry, and thus, difficult to be implemented in a wider variety of applications.
A CMOS APS detects (or amplifies) the signal within the sensor cell to greatly reduce the noise in determining the signal. However, the circuit in a typical CMOS APS sensor cell consumes an area, resulting in a reduced fill factor and low sensitivity. Another typical drawback associated with a CMOS APS sensor is high reset noise. A CCD sensor can allocate a large area for the light-sensing element, since the amplifiers and detecting circuits are not in the image sensor cell, when a double correlated sampling circuit is implemented. Thus, a CCD sensor typically has a large fill factor and high sensitivity. However, the transistors for correlated double sampling on a CMOS APS sensor can further reduce the sensor fill factor. Thus, many CMOS APS sensors using none correlated double sampling to balance the need for a large fill factor and reduced reset noise.
Although a CMOS image sensor, fabricated using the related simple CMOS process, typically has low power consumption, single power supply and the capability of on-chip system integration, in contrast with CCD image sensors, CMOS image sensors has not been yet widely used in image capture application because of low sensitivity and high noise.